1. Field
Disclosed are methods and circuits for adjusting the time constant of an LC circuit to maintain stable electron energy and minimize power losses in an accelerator device, such as a Betatron.
2. Description of the Related Art
Betatrons are magnetic devices used to accelerate electrons to relativistic energies. A high energy electron beam is extracted and directed on a suitable target, generating high energy x-rays. One application of the high energy x-rays is for logging oilfield boreholes, such as to map subsurface density and lithology.
Effective operation of a Betatron requires high, pulsed, currents and voltages to generate the magnetic field necessary for acceleration and confinement of the electrons. The Betatron device is controlled and run by several power supplies, which form the Betatron modulator. A conventional Betatron driving circuit utilizes a high voltage D.C. power supply, coupled to a pulse generating modulator circuit, which in turn drives the Betatron coils. U.S. Pat. No. 5,077,530 to Chen discloses a Betatron driving circuit having a combination of a low voltage D.C. power supply and a high voltage excitation capacitor to drive the Betatron. U.S. Pat. No. 5,077,530 is incorporated by reference herein in its entirety.
Proper timing is needed to maintain stable electron energy and to minimize power losses. The time constant, τLC, is the time required for the current to rise from a base value to a peak value during a single duty cycle:τLC=t1-t3  (1)As illustrated in FIG. 3, t1 is the time when current begins to rise and t3 is the time peak current is reached. A typical τLC for a Betatron modulator on the order of 20-40μseconds. τLC is mainly determined by the time constant combination L and C (reference numerals 10 and 12 in FIG. 1).τLC=√{square root over (LC)}  (2)
Varying either L or C, controls the length of this time interval. However, the LC time constant is affected by manufacturing tolerances and by component variations as a function of temperature. Temperature variations in a borehole are particularly extreme and may differ by on the order of 250° C. from the surface to the bottom of the borehole. Because of this, existing Betatron modular circuits cannot be accurately tuned to the precise time at which the peak of the current is reached and even if tuned at the surface, go off-peak due to temperature fluctuations. An additional source of heat that could increase the temperature variation is electrical resistance of the circuit components.
One way to tune an LC circuit is by using a capacitor bank with different value capacitors and a switch to connect the appropriate capacitor (or combination thereof) in series with or parallel to inductor L. Another way to tune LC circuits is disclosed in U.S. Pat. No. 6,121,850 to Ghosal that discloses a digitally adjustable inductive element utilized to provide a tunable oscillator.
A tunable inductor is disclosed in U.S. Pat. No. 5,426,409 to Johnson, which is incorporated by reference herein in its entirety. The tunable inductor has a magnetically saturable core with a pair of outer limbs joined to a center limb by connecting limbs. Signal winding are formed about each outer limb and a signal source connected to the signal windings induces a signal flux in the core through the outer limbs. A bias winding formed around the center limb induces a variable bias flux into the core. The flux induced by the signal windings is maintained below saturation. Adding flux via the bias winding controllably changes the inductance of the variable inductor. By applying a varying control current, we can move to a different section of the B-H curve, thereby obtaining a different amount of change in flux density provided by the same amount of variation of magnetization force.